The ICAO aircraft CO2 standard currently has only a small number of aircraft types in Europe certified and only certifies aircraft to CO2 metric value - which cannot be used to estimate full flight fuel consumption.
The ICAO CO2 calculator produces estimates of CO2 per passenger but does not utilise direct reporting for either fuel consumption or for traffic and as such serves a different purpose to the FEL. However, its methods of calculation and estimation have been agreed as a standard and the FEL is aligned in the methods where they are not in direct conflict with current and developing EU legislation.